Over the years, concerns about the steady decline of the available supply of petroleum in the face of ever-increasing demand for this valuable commodity have fueled researchers to look for possible alternative energy sources as well as chemical feedstock. One such resource is biomass. Biomass is a renewable, CO2-neutral and readily available source of energy. According to Oak Ridge National Laboratory estimates, about 512 million dry tons of biomass per year can be initially available at less than $50/dry ton delivered. This amount is equivalent to 8.09 Quads of primary energy. Currently, only 72 million dry tons (1.2 Quads) are being used for fuel. This leaves quite a margin that can still be exploited for energy production. Putting this into perspective, 6.89 Quads is about 1.2 B barrels of oil—equivalent to about 27% of the US net import of crude oil in 2004.
Hydrogen has been identified as a potential energy source, in addition to its role as an important intermediate in the chemical and petrochemical industry. Presently, most technologies rely on fossil fuels as the source of hydrogen production. Considering concerns about the future supply of petroleum, extensive research on alternative hydrogen generation technologies is currently underway. Several processes have been explored to produce hydrogen from biomass. One of the existing approaches involves the pyrolysis of biomass, followed by subsequent reforming of the bio-oil produced. Another technology is biomass gasification. However, these processes require very high temperatures (673 K-1100 K) even in the presence of catalysts and different gasification agents. A hydrogen yield of about 26 and 32 mmol/g sawdust has been reported in the gasification of aspen and cedar wood, respectively, at 2.4 wt % impregnation with CaO. They reported that the presence of CaO caused a decrease in the temperature from ˜850° C. to ˜675° C. at which the hydrogen production rate was at a maximum.
Hydrothermal processing of biomass and related model compounds has also been reported in the literature as a possible technology for hydrogen production. Modell in 1985 reported the complete solubility of maple sawdust in supercritical water. Sinag et al. also studied hydropyrolysis of glucose in supercritical water. In the presence of K2CO3 at supercritical water conditions, they reported high yields of CO2 and H2 with low yields of CO. Degradation compounds identified in the liquid by-product included furfural, phenols, and acids. The group of Antal et al. also studied the hydrothermal processing of wood, suggesting mechanisms for the formation of degradation products from wood carbohydrate constituents.
In 2002, Dumesic et al. first reported aqueous-phase reforming (APR) of oxygenated compounds that were chosen as model biomass mimics. They demonstrated the capacity to produce hydrogen in a flow reactor at temperatures (˜500K) much lower than those required for either pyrolysis or gasification. They reported that APR of molecules more reduced than sugars (methanol and ethylene glycol) had the highest hydrogen selectivity. Dumesic's group considered two reactions to be of major importance for hydrogen evolution: the reforming reaction (C—C cleavage),CmHnOmmCO+n/2H2  (1)and the water-gas shift (WGS) reaction,CO+H2OCO2+H2  (2)
Lignocellulosic biomass is chemically complex, consisting of cellulose, hemicellulose, lignin, extractives and inorganic materials. The first three groups are the main constituents, comprising as high as 98% of the material by weight. Both cellulose and hemicellulose are polymeric carbohydrates. The former (FIG. 2a) is a linear homopolymer of β-D-glucose linked by β-1→4 glycosidic linkages with high degrees of polymerization (DP). Cotton fibers contain the purest naturally occurring cellulose—with DP reaching as high as 10,000. The linearity of cellulose allows it to be compactly packed in crystalline regions in the cell wall. However, a small portion of the chains may become disordered and have a more random arrangement. These amorphous regions increase the susceptibility of cellulose to solvents and reagents.
The other carbohydrate group, collectively known as hemicelluloses, includes heteropolysaccharides of glucose, galactose, mannose, xylose and arabinose. Hemicelluloses are branched, with chains that are much shorter than cellulose (DP=150-250). Because of this, they are more susceptible to chemical degradation. Aside from the sugar components, some hemicellulose fractions may also be in acetylated or in uronic acid forms. FIG. 2 shows the two most abundant groups of hemicelluloses in plants. Mannans (FIG. 2b) are the major component of softwoods while xylans (FIG. 2c) are the main constituent of hardwoods. Lignin (FIG. 2d), on the other hand, is phenolic and structurally more complex than the other component. To date, all studies of APR have focused on the use of model compounds that could be derived from biomass, such as sorbitol and ethylene glycol—no one has yet reported the APR of actual biomass for hydrogen production.
Though biomass-derived energy is yet to be fully demonstrated as an economically viable alternative to fossil fuel, the application of this process directly to readily available biomass reserves may prove to be more attractive in the long run when petroleum becomes increasingly more scarce and expensive. So far, results of hydrogen production studies using APR on compounds such as ethylene glycol have been encouraging. However, due to its varied composition, we expect that APR of lignocellulosics would be more complicated than when using these representative compounds. The chemistries would certainly be different, with functionalities present in biomass that are absent in the model compounds. Reforming of biomass also introduces a solid phase into a previously completely aqueous phase feed. We also anticipated that unlike the previous studies, breakdown of polymeric structures to simpler molecules would be necessary for catalytic reforming to occur.